This invention relates generally to surface and near surface optical treatments and specifically to optical surface treatments for the purpose of reducing susceptibility to optical damage (i.e.. increasing the surface damage thresholds with respect to fluence [Joules/cm.sup.2 ] and intensity [Joules/cm.sup.2 .cndot.sec]).
Contamination of optical surfaces of optical elements by impurities of any origin, for example, particulate debris, adsorbed volatile substances, or environmentally induced chemical derivatives of the parent material, can serve as nucleation sites (microscopic origins of macroscopic damage) under intense optical illumination. Damage is initiated at the nucleation sites due to optical inhomogeneities (variations in the optical properties of the nucleation sites with respect to those of the surrounding parent material), especially optical absorption. In optical absorption, optical energy is converted to highly localized thermal energy that gives rise to stresses exceeding the fracture limit or that chemically decomposes the parent material. Where chemical decomposition results from the interaction between the optical energy and the nucleation sites, a cascade effect can result in which the decomposition products show further increases in optical absorption, and continued optical illumination supports a destructive chain reaction.
Processing the surfaces of optical materials, coating or polishing for example, whose intrinsic damage resistance (the damage resistance of the pure material) is high, to meet optical specifications, must account for the potential of process-induced nucleation sites that reduce the optical damage resistance of the finished optical element. Processes that induce low damage resistance impurities must be modified to reduce or eliminate such induced impurities. One example of such a process is the use of a polishing slurry that reacts chemically with the parent material or results in a change in the structure of the material (e.g. induces a transformation from one crystal structure to another).
Accordingly, there is a desire in the art for a method of optical surface treatment that reduces or eliminates induced impurities.